A multiple-input multiple-output (MIMO) wireless communications system includes a plurality of antenna elements at the transmitter and a plurality of antenna elements at the receiver. A respective antenna array is formed at the transmitter and at the receiver based upon the antenna elements associated therewith.
The antenna elements are used in a multi-path rich environment such that due to the presence of various scattering objects in the environment, each signal experiences multipath propagation. The receive antenna elements capture the transmitted signals, and a signal processing technique is then applied to separate the transmitted signals and recover the user data.
The signal processing technique may be a blind source separation (BSS) process. The separation is “blind” because it is often performed with limited information about the transmit signals, the sources of the transmit signals, and the effects that the propagation channel has on the transmit signals. Three commonly used blind signal separation techniques are principal component analysis (PCA), independent component analysis (ICA) and singular value decomposition (SVD).
MIMO communications systems are advantageous in that they enable the capacity of the wireless link between the transmitter and receiver to be improved. The multipath rich environment enables multiple orthogonal channels to be generated therebetween. Data for a single user can then be transmitted over the air in parallel over those channels, simultaneously and using the same bandwidth.
Current MIMO communications systems use spatially diverse antenna elements so that the number of orthogonal channels that can be formed is not reduced. The problem with such an implementation is that the performance of a MIMO communications system is usually proportional to the number of antenna elements used.
Increasing the number of antenna elements increases the size of the antenna arrays for MIMO communications systems. When a MIMO receiver is implemented within a small portable communications device, there is little available volume for a large number of antenna elements, and mounting the antenna elements on the outside of the communications devices is a problem for the user.
One approach for providing a more compact antenna array for a MIMO receiver is disclosed in U.S. Pat. No. 6,870,515. Instead of using spatially diverse antenna elements, polarization diversity is used. Since closely spaced antenna elements are used, this enables a compact antenna array to be provided for a MIMO receiver.
Even though a more compact antenna array is provided, performance of the MIMO communications system is still based on the number of antenna elements at the receiver being equal to or greater than the number of antenna elements at the transmitter. For example, the '515 patent discloses that the number of receive antenna elements is equal to or greater than the number of transmit antenna elements.
In addition, if two or more received signals are close together in angular distance, generation of different antenna patterns by the MIMO receiver may not be adequate to determine differences in the received signals. Even if beam forming is used, making the beam sufficiently narrow or the bore sight adjustable may not be practical or cost effective. Consequently, there is a need to retain some antenna elements to differentiate these signals.